I’m having problems breaking the 12 m ceiling on my long run. I feel fine up to 10 miles then I start getting dizzy & sick. I load up on carbs the day before, I eat sports beans after 4 miles then 1 ever 2 miles and I drink 8 ozs G2 every 2 miles. I ran with just water before up to 10 miles and did fine. Thought I had this figured out as low blood sugar, so I increased my carbs on the run. I’m 50 and I’ve been running for 2 yrs. I recover quickly, I run early and I’m done by around 9 AM±. I’m running 34-36 mpw. Any recommendations? I’m listening.
Run with a running partner, it’ll get your mind off of things and make the time go by faster. Try some sport gels. I use Crank Bros. every 5 miles, easily asorbed. Gu’s are ok but you take them in fewer miles. Last but not least, try walking for 1 minute per mile run. You’ll be surprised how much juice you’ll have left.
What pace are you running for your long runs and what are some recent race times?
With a normal diet, you really don’t need much more than water for a 12 mile run.
First off, ditch the G2, you do not want to “consolidate” fuel sorces and you also do not want to mix simple sugars (G2 and sport beans). Find one that your stomach can handle and stick with that.
What time are you doing your carb load the day before? Breakfast is the only meal that will allow the time for those carbs to be stored in a usable fashion. If you are stuffing your face with spaghetti at 6pm the night before your run, you will only be fueling your glycogen stores that you will use up quickly.
I highly doubt the problem has to do with what you eat. I bet your problem has to do with poor hydration and hyonatremia(lack of electrolytes), as well as all that sugar in your stomach (i know that makes me naseous). Look into S! Caps or Salt Sticks for your electrolye fueling. Purely what you need, and nothing else. Stick with water, and plenty of it, and salt sticks. Maybe a sport bean pack after an hour or so to give you a little pick me up.
What is your weekly mileage? What kind of temperature are you running in? There could be several factors but I would start with hydration and fueling. I think maybe you are overdoing it for that short of an effort.
Bearclaw, When I say sick, it’s the feeling that I get when I’m over heated, I have done that before by running in the noon heat and I know that feeling, but the temp was only about 85 degs when I quit. Carb loading noon and 6pm the day before. Temp was 75 when I started and 85 when I stopped (ambient), I don’t know what it is on the black top. Humidity 48%. Mileage last week was 36 miles.
I did an experiment this AM. Weighed in before the run at 220 lbs. Ran 8 miles and drank 8 ozs water every 2 miles. At the end I weighted 215 lbs. I am dehydrating. Any other thoughts are appreciated or point me to a running guide for dummies. 
I don’t think the dehydration is the main issue. I’ll lose 5 - 6 pounds on some of my longer runs and I’ll feel fine (and this is at 140lbs).
Your symptoms sound a lot like heat exhaustion - maybe compounded by not taking enough fluids. When it gets as hot as it is out there now… the body just can’t do it all. The more energy you spend running, the less energy the body has to cool itself. And the only way to push through it is to slow WAY down, take walk breaks, and keep your temperature down.
Something I do in the summers is to concentrate more on the amount of time running rather than distance or pace. Put in the time and your body will respond. 
Can you elaborate on that and cite your sources? My running coaches always told us to carb overload the night before so they would be ready for a run the next morning. I always saw great results following a carb overload the night before. Mental? Maybe.
If you are regularly running 10 miles then it is not carbs or anything like that keeping you from breaking 12 miles. It could be you are starting too fast. Read the Lore of Running for a detailed analysis of how energy is stored and extracted from muscles, blood, fat, etc. And the effect of starting to fast has on the efficient pull of energy from them.
Basically you want to start at a slower/moderate pace for the first 20 minutes to make sure your body switches to endurance mode.
Also, anything above 75F you should be careful about hydration and electrolytes. For me, any run under an hour I just use water. For up to 2 hours I would use Gatorade at about 20 oz per hour (from the start with a few gulps every 20 minutes to equal 20+ oz per hr).
Beyond 3 hours I would be snacking and drinking every 20 minutes to consume one Clif Bar and 20 oz. electrolyte over hour.
With basic hydration and electrolytes in place I would bet your issue is more about starting too fast and fading at the end. Also, I think it is better, if needed, to take a walk break every half hour or so to ensure you let your body catch up energywise and therefore run longer, than to run until you drop.
You can’t run 15 miles the first time at the same pace you run 8 miles. When you run distance, the distance itself is far more important than the time. Long runs should be slow. You work on speed on the shorter runs under 10 miles, intervals, etc.
The first tiem I trained for a marathon, I hit the wall at 13 miles. It was like the energy pump just shut off and I could barely move my legs. My next long run I hit it around 15 miles, then 18 miles. In the marathon itself i hit it around 21 miles. Afterward I was able to build up to 30 mile trail runs. Your body will adjust to longer runs but you have to give it more time the first time.